1. Field of the Invention
The present invention relates to a signal transmitting method and apparatus, and more particularly, to a signal transmitting method and apparatus by which an orthogonal frequency division multiplexing (OFDM) method is improved.
2. Description of the Related Art
As a transmission rate is increased when data is transmitted through a wire or wireless channel, multipath fading or intersymbol interference (ISI) is increased, so that reliable data transmission cannot be expected. Orthogonal frequency division multiplexing (OFDM) and discrete multitone (DMT) are resistant to the multipath fading and ISI and their band efficiencies are high, so that they are adopted in the signal transmitting method of a digital audio broadcast (DAB) and digital television (TV) in Europe, and they are used for an asymmetric digital subscriber line (ADSL) and a universal asymmetric digital subscriber line (UADSL) in U.S.A.
FIG. 1 shows a typical OFDM signal transmitting procedure. A series of input data bits bn is encoded to sub-symbols Xn by an encoder 102. A series of Xn is converted to N-sized vectors or blocks by a serial-to-parallel converter 104. A pilot tone adder 105 adds M pilot tones Pi (i=1, . . . , M) to Xn to achieve channel estimation in a receiving side. The output of the pilot tone adder 105 is N-point inverse fast Fourier transformed by an N-point inverse fast Fourier transformer (N-IFFT) 106, to N time domain signal xk. Here, n indicates a frequency domain index, and k indicates a time domain index.                                           x            k                    =                                    1              N                        ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  n                                ⁢                                  ⅇ                                      j2π                    ⁢                                                                                  ⁢                    kn                    ⁢                                          /                                        ⁢                    N                                                                                      ,                  k          =          0                ,        …        ⁢                                  ,                  N          -          1                                    (        1        )            
A parallel-to-serial converter 108 transforms the vectors or blocks composed of N elements to a series of time domain signals xk. A cyclic prefix adder 110 copies the last G signals from the N signals and attaches them to the front of the N signals. The G signals are referred to as cyclic prefix. (N+G) signal samples compose an OFDM symbol block in a time domain. The OFDM symbol block is consecutively converted to analog signals through a digital-to-analog converter 112, and the converted analog signals are output after an intermediate frequency (I/F) process and a radio frequency (R/F) process. The above-described procedure is typical for signal transmission in an OFDM system. Here, the position of the encoder 102 may be exchanged with the position of the serial-to-parallel converter 104.
FIG. 2 shows a typical procedure for receiving OFDM signals. The received analog signals are converted to a base band signal r(t) through an R/F process and an I/F process, and the analog signals are sampled through an analog-to-digital converter 202 to convert the base band signal r(t) to a digital signal rk. A cyclic prefix remover 204 detects the starting of the OFDM symbol block from the received signals to remove the cyclic prefix, and then outputs N signal samples. The serial-to-parallel converter 206 converts a series of signal samples to N-sized vectors or blocks and outputs the N-sized vectors or blocks to an N-point fast Fourier transformer (N-FFT) 208. The N-FFT 208 transforms time domain signal rk to a frequency domain signal Rn.                                           R            k                    =                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                          r                k                            ⁢                              ⅇ                                                      -                    j2π                                    ⁢                                                                          ⁢                  kn                  ⁢                                      /                                    ⁢                                                                          ⁢                  N                                                                    ,                  n          =          0                ,        …        ⁢                                  ,                  N          -          1                                    (        2        )            
The Rn can also be expressed by the following Equation 3:Rn=Xn·Hn+In+Wn  (3)wherein Xn denotes data including data and a pilot tone Pi, Hn denotes a channel response, In denotes intercarrier interference, and Wn denotes additive white Gaussian noise (AWGN).
A channel estimator 209 can obtain M channel responses from the output Rn of N-FFT 208 using the already-known pilot tone Pi as in Equation 4:                                                         H              .                                      n              ,              i                                =                                                    R                                  n                  ,                  i                                                            P                i                                      =                                          H                                  n                  ,                  i                                            +                                                [                                                            I                                              n                        ,                        i                                                              +                                          W                                              n                        ,                        i                                                                              ]                                                  P                  i                                                                    ,                                  ⁢                  i          =          1                ,        …        ⁢                                  ,        M        ,                  n          =          1                ,        …        ⁢                                  ,        N                            (        4        )            
The channel estimator 209 estimates a channel distorted by linear interpolating a channel response of data symbols, from {dot over (H)}n,j.
A frequency domain equalizer (FEQ) 210 compensates for signal deformation generated by the channel with respect to the output Rn of the N-FFT 208, using the output of the channel estimator 209 as the tap coefficient of the FEQ 210 for each frequency index n.
A detector 212 detects an original sub-symbol {circumflex over (X)}n from the output Zn of the FEQ 210. The parallel-to-serial converter 214 converts the N-sized vectors to a series of signals, and a decoder 216 decodes a bitstream of data {circumflex over (b)}n. The above-described processes are typical for receiving signals of the OFDM system. Here, the position of the parallel-to-serial converter 214 may be exchanged with the position of the decoder 216. Also, the detection by the detector 212 and the decoding by the decoder 216 may be performed in one step.
Several sub-symbols Xn are added as shown in Equation 1, so that the time domain OFDM signal xk has a Gaussian distribution according to the central limit theorem. As a result, the peak-to-average power ratio (PAR) of the signal is very high.
FIG. 3 shows the amplitude of the time domain OFDM signal when N=256 and Xn is a quadrature phase shift keying (QPSK) symbols. When the PAR is high, clipping or severe quantization noise may occur in the digital-to-analog converter of a transmission terminal. When signals are transmitted, clipping and non-linear distortion may occur in a high power amplifier (HPA) of the R/F stage to thereby rapidly deteriorate performance. If a HPA is restricted to operate at a low power intentionally to avoid this problem, the efficiency of the HPA and total system performance can be deteriorated.
The PAR of a jth OFDM symbol xj,k is defined as follows.                               ζ          j                =                                            max                              0                ≤                k                ≤                N                                      ⁢                                                        x                                  j                  ,                  k                                                                                        σ            x                                              (        5        )            
The peak power of the time domain OFDM signal is different in every symbol, so that the PAR can not be obtained beforehand and only the statistical characteristics can be obtained. FIG. 4 shows the probability Pr{ζj>ζ0} of the PAR values of the OFDM system being higher than a predetermined value ζ0 when N is changed to 2, 4, 8, 16, . . . , 1024.
A maximum PAR generated by the OFDM system can be easily obtained through Parseval's theorem. The maximum PAR generated by the OFDM signal having N sub-symbols is as follows.                               ζ          j                =                                            max                              j                ,                                  0                  ≤                  k                  ≤                  N                                                      ⁢                                                        x                                  j                  ,                  k                                                                                        σ            x            2                                              (        6        )            
Here, σx2 indicates variance of the time domain signals.maxj,0≦k≦N|xj,k|=maxn|Xn|=C  (7)σx2=σx2/N  (8)
Here, σx2 is the variance of the frequency domain signal Xn, and maxn|Xn|=C can be obtained by the constellation of the sub-symbol Xn. Thus, the PAR of Equation 5 can be obtained as follows.ζ=√√{square root over (N)}ζX  (9)
Here, ζX=C/σX indicates a PAR of the given sub-symbol Xn, and is also the PAR of the conventional single carrier method since this method allows symbols to be transmitted without any conversion. Thus, Equation 9 shows a difference in PARs between the signal of the OFDM system obtained by the multi-carrier method and the signal of the conventional single carrier method.
In the conventional OFDM system, an N-point IFFT/FFT is used, so that the PAR of the signal is very great. Thus, various methods for reducing the PAR of the OFDM signal have been developed. The conventional algorithm for reducing the PAR of the OFDM signal is simple and very effective when the size N of the OFDM symbol is small, however, inappropriate for when N is large. In the algorithm adopted when N is large, as the PAR decreases much, the complexity and the information loss are increased.
Methods for reducing the PAR of the OFDM signals using coding are disclosed in papers “Block Coding Scheme for Reduction of Peak to Mean Envelope Power Ratio of Multi-Carrier Transmission Schemes”, Electronics latters, vol. 30, No. 25, pp. 2098˜2099, December 1994, by A. E. Jones, T. A. Wilkinson and S. K. Barton, “Simple Coding Scheme to Reduce Peak Factor in QPSK Multicarrier Modulation”, electronics letters, vol. 31, No. 14, pp. 113˜114, July 1995, by S. J. Shepherd, P. W. J. van Eetvelt, C. W. Wyatt-Millington and S. K. Barton, and “OFDM Codes for Peak-to-Average Power Reduction and Error Correction”, proc. of Globecom '96, pp. 740˜744, London, November 1996, by Richard D. J. van Nee. But, the above methods cannot be adopted for an OFDM symbol for which N is greater than 16.
A reduction in noise by a reduction in the amplitude of a signal is obtained by U.S. Pat. Nos. 5,787,113 and 5,623,513 “Mitigating Clipping and Quantization Effects in Digital Transmission Systems”, and papers “Mitigating Clipping Noise in Multi-Carrier Systems”, proc. of ICC, '97, PP. 715˜719, 1997. But, the above method requires a reduction in the amplitude of the signal, so that the signal to noise ratio of the receiving terminal is reduced, and the reduction in the PAR is not great.
In U.S. Pat. No. 5,610,908 entitled “Digital Signal Transmission System Using Frequency Division Multiplexing”, the phase of a desired frequency domain signal is restored to an initial phase, and signals around band edges are attenuated, in order to reduce the peak power value. However, this method is disadvantageous in that as peak power is reduced, more-information is lost.
A method for determining a value appropriate for a redundant frequency index to eliminate the peak of a time domain OFDM signal is disclosed in references “Clip Mitigation Techniques for T1.413 Issue3”, T1E1. 4/97-397, December 1997, by Allan Gatherer and Michael Polley, and “PAR Reduction in Multi-Carrier Transmission Systems”, T1E1.4 VDSL, T1E1.4/97-367, Dec. 8, 1997, by Jose Tellado and John M. Cioffi. Here, in order to increase reduction in peak power, the redundant frequency must be increased and thus information loss must be increased.
Two methods for changing the frequency domain phase of the OFDM signal to reduce the time domain peak power are compared in reference “A Comparison of Peak Power Reduction Schemes for OFDM”, proc. of Globecom '97, pp. 1–5, 1997, by Stefan H. Muller and Johannes B. Huber. By these methods, a hardware configuration becomes complicated because various N-point IFFTs should be simultaneously performed. Information loss can be generated because phase change information must be transmitted together with data. Information in the phase change must be exactly detected by the receiving terminal.
In the conventional single carrier transmission method, PAR is not great and thus the above-described problems of the OFDM system are not generated. By the conventional single carrier method, an equalizer is trained and operated in the time domain. When the data transmission rate is increased, signal interference by a channel is rapidly increased, so that the number of equalizer taps of the receiving unit must be increased. At this time, the training of the equalizer requires much time and the operation thereof is complicated. However, the FEQ of the OFDM system is trained and operated in the frequency domain, where one tap is required per frequency and training and operation are very simple. Thus, the OFDM method is appropriate for high-speed data transmission. But, the PAR of the OFDM signal is great so that it is difficult for the OFDM method to be utilized.
In the OFDM system, a transmission signal received via channel, is distorted by the characteristics of the channel and the influence of AWGN or the like, so that an accurate channel estimation is required to detect a transmitted signal from the distorted received signal. In particular, under a channel environment having severe fading, a channel changes more rapidly, so much transmission information cannot be decoded if the channel is not properly estimated.
A. Leke and John. M. Cioffi [“Impact of Imperfect Channel Knowledge on the Performance of Multicarrier Systems”, GLOBECOM'98] discloses a signal-to-noise ratio (SNR) in the case in which an accurate channel estimation is not achieved in the OFDM system, and emphasizes the importance of channel estimation. However, this paper does not mention the details of a channel estimation method.
An existing method for estimating a channel in the OFDM system includes a method using a reference symbol or a method using a pilot tone. U.S. Pat. No. 5,771,223, entitled “Method of Receiving Orthogonal Frequency Division Multiplexing Signal and Receiver Thereof” discloses a method using a reference symbol, and U.S. Pat. No. 5,771,224, entitled “Orthogonal Frequency Division Multiplexing Transmission System and Transmitter and Receiver Thereof” discloses an invention for estimating a channel using a reference symbol and a null symbol. The method of estimating a channel using a reference symbol is appropriate for an environment having little change in the characteristics of the channel, but causes many channel estimation errors in a channel environment having severe fading.
In U.S. Pat. No. 5,406,551, entitled “Method and Apparatus for Digital Signal Transmission using Orthogonal Frequency Division Multiplexing”, a pilot tone is added to data at regular intervals and transmitted in an already-known frequency domain, and, in the receiving stage, the pilot tone is detected in the frequency domain, and the degree of attenuation of a channel is estimated by linear interpolation and compensated for. Linear interpolation, which is a general channel estimation method using a pilot tone, is suitable for the environment where a channel change is slow. However, when the channel change becomes severe, much fluctuation occurs between pilot tones added at regular intervals in the frequency domain in the transmitting stage, thus making a channel estimation error worse. This problem can be overcome by U.S. Pat. No. 5,774,450 entitled “Method of Transmitting Orthogonal Frequency Division Multiplexing Signal and Receiver Thereof”, paper of F. Tufvesson and T. Maseng [“Pilot Assisted Channel Estimation for OFDM in Mobile Cellular Systems, Vehicular Technology Conference, 1997], paper of M. J. F. Garcia, J. M. Paez-Borrallo and S. Zazo [“Novel Pilot Patterns for Channel Estimation in OFDM Mobile Systems over Frequency Selective Fading Channels”, PIMRC, 1999], which disclose deformation of a pilot tone to reduce channel estimation error in an environment having a serious channel change while using linear interpolation.
U.S. Pat. No. 5,912,876, entitled “Method and Apparatus for Channel Estimation”, and U.S. Pat. No. 5,732,068, entitled “Signal Transmitting Apparatus and Signal Receiving Apparatus using Orthogonal Frequency Division Multiplexing”, are related to an encoded pilot signal generator and a clock/pilot signal generator, respectively. In these references, a pilot signal is generated by encoding, and is added to the time domain output signal of an N-IFFT in the transmitting step and transmitted. These references can obtain a better channel estimation performance than other inventions because of a margin obtained by encoding. However, these references do not consider the PAR at all, so that many problems are generated in actual system implementation.